Most electrochemical cells include a negative electrode, a positive electrode and an electrolyte providing passage for the ionic electro-active species of the electrochemical cell. Electrolytes may be solid or liquid or a composite of both. The electrodes are usually prevented from coming into direct contact by some form of a separator or solid electrolyte, which allows the movement of ionic electro-active species but not of electrons. The separator in some type of batteries provides physical support to the solid electrolyte. Electrochemical cells or batteries are usually equipped with current collectors which can be connected to an external electrical circuit for utilizing the electrical energy generated by the battery. In case of rechargeable electrochemical cells or batteries, the same current collectors serve in the recharging of the battery or cell.
In the last decade or more lithium batteries have been developed for generating electrical energy. Rechargeable lithium batteries may be cylindrical or button shaped and in such forms they often have a non-aqueous liquid electrolyte. More recently, thin plate rechargeable lithium batteries have been developed which are suitable for use in electronic devices of current design, as well as having high energy density per volume or weight. Rechargeable thin plate lithium cells or batteries often utilize as the anode active substance, lithium foil or lithium alloy, or a substance capable of reversibly intercalating lithium ions. The cathode of a rechargeable lithium battery usually contains a transition metal chalcogenide or equivalent, as the positive active material.
The electrolyte of a thin plate rechargeable lithium battery may be a solid electrolyte laminate containing lithium ions, or alternatively, the electrolyte may take the form of a separator sheet in which a non-aqueous solution containing the electro-active component, that is a compound bearing a dissociable lithium ion, is dispersed. Separators for lithium batteries are frequently formed of inert, porous or microporous polymer layers or sheets, which are subsequently impregnated with a liquid electrolyte containing a dissolved lithium salt or similar dissociable lithium bearing substance. The polymer sheet either as a solid electrolyte or as host for a liquid electrolyte, needs to be durable and strong to render effective barrier between the electrodes, as well as to be able to supply sufficiently high concentration of mobile, electro-active species per unit area or unit volume for yielding high current density. It can be seen that the development of suitable electrolytes is a very important aspect of thin film, rechargeable lithium battery technology.
As is known, the role of the electrolyte is to provide ionic communication between the electrodes within the battery. Simultaneously, the separator performs two functions: it maintains a physical and electronic barrier between the electrodes while it provides pathways for the ionic electro-active species for the necessary communication between the electrodes.
Conventional solid, polymer electrolyte compositions incorporate dissociable lithium ion bearing compounds in their structure. The mobility of the electro-active species in the polymer matrix will depend on the nature of the lithium compound having labile lithium ions, as well as on the temperature of the lithium battery operation and such like. It is noted that the mechanical strength of polymers capable of incorporating dissociable lithium ion bearing compounds is often low, and may also be subject to degradation by the electrode materials if the temperature of the battery rises above normal operating temperatures. The lack of mechanical strength may require that solid polymer electrolytes have substantial thickness, which may lead to diminished energy density per unit volume for lithium batteries. The ionic conductivity of lithium ion conducting solid polymer electrolytes is usually in the range of 10−4 to 10−2 S/cm.
Hybrid electrolytes for thin plate rechargeable lithium batteries often utilize organic solvents or mixtures thereof for the dissolution of a lithium compound. There are known solvents or mixtures of solvent compounds, such as disclosed, for example, in U.S. Pat. No. 5,643,695 issued to Barker et al. on Jul. 1, 1997. As briefly referred to above, an hybrid lithium battery electrolyte has an inert, porous separator layer for keeping the electrodes separated, and to hold in its pores and micro-pores a large reservoir of dissociable lithium ions for enabling the lithium battery to generate high current density. The lithium battery may be assembled of a negative electrode layer, a positive electrode layer and an inert, plasticised separator layer between the electrodes. The plasticizer may be, at least in part, replaced by an organic lithium ion solution before packaging the battery, as is described in U.S. Pat. No. 5,456,000, issued to Gozdz et al. on Oct. 10, 1995. Inert polymer separators composed of multiple layers of polyolefin membranes of different porosity and melting point, are described in U.S. Pat. No. 4,650,730, issued to Lundquist et al. on Mar. 17, 1987. It is noted that most known separator sheets are inert, in other words, only the electro-active components of the organic solution retained in the cavities and pores of the separator layer take part in the cell reaction. High pore density of the separator sheet may provide a high population of electro-active species but it may also undermine the mechanical strength, and hence the durability of the hybrid electrolyte.
More recently composite, hybrid electrolytes for use in rechargeable lithium batteries have been described, wherein the separator is impregnated and/or coated with an inert gel of organic, long chained, uncured, polymerizable composition, which is capable of absorbing lithium ions or mobile lithium ion bearing compounds. It is noted that in several, conventional lithium electrochemical cell electrolytes the organic, long chained, polymerizable, absorbent composition, which is coating, and/or is adsorbed on the faces, as well as is filling completely the pores of the inert separator, does not contain lithium ions at the time of assembling the electrodes and the coated inert, porous or micro-porous separator. The electrolyte, which is a lithium ion containing organic solution, however, is added at subsequent stages, followed by the polymerisation or curing of the organic, long chained, absorbent polymer coating. Such multi-layered polymer systems are described in U.S. Pat. Nos. 5,658,685, 5,681,357, 5,688,293 and 5,716,421, issued to M. Oliver, Eschbach et al., Oliver et al. and Pendalwar et al, on Aug. 19, 1997, Oct. 28, 1997, Nov. 18, 1997 and Feb. 10, 1998, respectively. In the multi-layered, polymer systems for use in the lithium batteries referred to above, the inert, porous polymer separator is a polyolefin layer, and the polymerizable gel is polyvinylidene fluoride (PVDF) or chemically equivalent polymer or copolymer. The gelling compound of the above publications, is supported by the inert, porous polyolefin layer, and is intended to serve as an inert absorbent for the lithium ion containing organic solution added subsequently. In the methods taught by Eschbach et al., Oliver et al. and Pendalwar et al. the gelling compound is cured and polymerized in the packaged and sealed battery by subjecting the package to heat and pressure, thus bonding the electrodes to the separator bearing the absorbent polymerizable gelling compound. The heat and pressure treatment which is required to solidify/polymerize the gelling compound of the lithium batteries made according to the above methods, may damage the packaging of the lithium battery so produced, thereby rendering the packaging more vulnerable to moisture and similar atmospheric damage. Moreover, the curing of the battery components subsequent to packaging and sealing, may generate undesirable gases and similar compounds detrimental to the satisfactory operation of the lithium battery. It is also noted, that in the above described, conventional multi-component, polymer electrolyte systems containing polymerizable or polymerised gelling compounds, there is only one kind of electro-active lithium species present, which is added to the multi-component, layered electrolyte subsequent to assembling the electrochemical cell.
Lithium batteries utilizing inert, porous, multi-layered, polymer separator sheets coated, and having the pores of the separator filled with a polymerizable gelling composition which can absorb compounds containing mobile electrolyte-active lithium, are described in U.S. Pat. Nos. 5,837,015 and 5,853,916, issued to Venugopal et al. on Nov. 17, 1998, and Dec. 29, 1998, respectively. It is noted that the above mentioned patents to Venugopal et al. contain only one species of electrolyte-active lithium compound, which is introduced into the assembled battery in the form of an organic, lithium compound bearing solution.
A method of manufacturing a tri-layered battery separator is described in U.S. Pat. No. 5,952,120, issued to Yu et al. on Sep. 14, 1999. Inert, porous polymer separators made of porous layers having different polyethylene and polypropylene blend compositions and hence different mechanical properties, are described in U.S. Pat. No. 5,856,039, issued to Masatoshi Takahashi on Jan. 5, 1999.
There is a need for an electrolyte system for use in thin plate rechargeable lithium batteries which has enhanced mechanical integrity and strength provided by a multi-layered inert, micro-porous separator, as well as capability of high ionic conductivity, without unwarranted increase in the thickness of the electrolyte layer.